The present invention relates to a novel sintered ceramic composite having a good thermal shock resistance well sufficient for practical use and improved in mechanical strength after suffering from thermal shock, melting loss resistance in a molten metal and adhesion resistance to a molten metal. The present invention further relates to a molten metal contact member produced from the sintered ceramic composite, which member is utilized at a portion where a thermal shock resistance, melting loss resistance and adhesion resistance are required.
As ceramics having a good thermal shock resistance, cordierite (MgO--Al.sub.2 O.sub.3 --SiO.sub.2 ceramics) and a sintered aluminum titanate, etc. have been generally known (JP-B-54-1564). These ceramics have a sufficient temperature difference for resisting a thermal shock, however, insufficient in a mechanical strength. Therefore, their use as a thermal shock resistant material has been largely restricted.
As other ceramics having a good thermal shock resistance, a sintered boron nitride, a sintered silicon nitride, a sintered sialon have been also known. However, the sintered boron nitride has a low mechanical strength and abrasion resistance, while having a good thermal shock resistance and a melting loss resistance in a molten metal. Tile sintered silicon nitride and the sintered sialon are inferior in a melting loss resistance in a molten steel while having a good thermal shock resistance, mechanical strength and abrasion resistance.
In order to compensate for the defects in each of conventional ceramics, ceramic composites comprising boron nitride and silicon nitride, or boron nitride and sialon have been reported. The ceramic composites of boron nitride and silicon nitride have been reported in JP-A-56-120575, JP-A-1-131062, JP-A-4-294846 and JP-A-5-70234. As the ceramic composites of sialon and boron nitride, JP-A-60-145963 discloses the ceramic comprising .beta.-sialon and boron nitride, JP-A-2-255247 discloses the ceramic comprising .beta.-sialon, electrofused alumina and boron nitride, JP-A-2-255248 discloses the ceramic comprising .beta.-sialon, zirconia and boron nitride, and JP-A-3-153573 discloses the ceramic comprising .beta.-sialon, boron nitride and glass phase. However, these ceramic composites have a drawback that the silicon-component contained therein, on contacting with a molten metal, especially with a molten iron, reacts selectively with iron to cause a melting loss as a result.
In order to improve the melting loss resistance of the conventional ceramics, a ceramic composite in which the melting loss resistance is improved by addition of aluminum nitride has been proposed. For example, a ceramic composites comprising boron nitride, aluminum nitride and silicon nitride is reported in JP-A-56-129666 and JP-A-60-51669. However, the improvement in the melting loss resistance is sill insufficient since the ceramics disclosed therein contain 10 wt. % or more of silicon nitride.
Recently, ceramic composites comprising boron nitride and aluminum nitride and containing no silicon nitride have been proposed. JP-A-1-246178 discloses a ceramic comprising boron nitride, aluminum nitride and Y.sub.2 O.sub.3. JP-A-1-131069 discloses a ceramic comprising boron nitride and aluminum nitride. JP-A-1-261279 discloses a ceramic comprising boron nitride, aluminum nitride and a calcium compound. JP-A-1-252584, JP-A-1-261279 and JP-A-1-305862 disclose ceramics comprising boron nitride, aluminum nitride, a calcium compound and an yttrium compound. JP-A-3-252367 discloses a ceramic comprising aluminum nitride, boron nitride and 3CaO.Al.sub.2 O.sub.3. Further, as ceramics for use with a molten iron-based alloy, JP-A-4-332831 and JP-B-5-8141 disclose a ceramic composite comprising boron nitride, aluminum nitride and yttrium oxide.
Aluminum nitride is well known as a highly heat-conductive material, and is superior as a structural material due to its hardness of the same level as alumina, good abrasion resistance and melting loss resistance to a molten metal. However, aluminum nitride is poor in mechanical strength and thermal shock resistance. Therefore, ceramic composites in which aluminum nitride is incorporated is deteriorated in its mechanical strength, particularly in mechanical strength after suffering from a thermal shock, while being improved in melting loss resistance.
In addition, aluminum nitride can be made into a material having a good thermal shock resistance, abrasion resistance and melting loss resistance to a molten metal by incorporating boron nitride. However, the ceramic composite of boron nitride and aluminum nitride contains boron oxide inherently contained in boron nitride and a rare earth metal oxide such as yttrium oxide which is used as a sintering aid. When such a ceramic composite is immersed into a molten metal in a long time period, a complex oxide derived from the molten metal and an oxide thereof leads to the wetting of the ceramics with the molten metal thereby causing a corrosion by the molten metal. Then, the ceramics finally suffer from a melting loss or an undesirable adhesion of the metal on the ceramic surface which may result in rupture of the ceramics upon peeling of the adhered metal. Therefore, for the ceramics usable as a molten metal member for a molten metal, it is insufficient to merely have a good melting loss resistance and it is necessary to have a good adhesion resistance to a molten metal. A ceramic having a sufficient adhesion resistance can be effectively used as a member, especially, as a member which is immersed into a molten metal because a small amount of or no adhesion of the metal can avoid the melting loss. In the present invention, the term of the adhesion resistance to a molten metal means that no metal adheres on the ceramic surface after a ceramic is immersed in a molten metal or means that a slight amount of metal adheres on the ceramic surface while the adhered metal can be easily removed from the surface without causing any rupture of the ceramic. In the case that a ceramic is highly adhesive to a molten metal or the adhered metal is hardly removed from the ceramic surface, the ceramic suffer from the melting loss or rupture. This results in a problem that the molten metal is deteriorated in its characteristics by the contamination due to ceramic fragments and the repeated use of the ceramic is made difficult.
Therefore, a ceramic which is superior in the melting loss resistance and the thermal shock resistance, especially, in the bending strength after thermal shock has been strongly desired to be developed. In addition, a ceramic which is superior not only in the thermal shock resistance and the melting loss resistance but also in the adhesion resistance to a molten metal has been also desired to be developed.